1. Field of the Invention
The present invention generally relates to regenerative cryocoolers and pulse tube cryocoolers, and more specifically, to a regenerative cryocooler, such as a pulse tube cryocooler, configured to cool a subject to be cooled at a cryogenic temperature.
2. Description of the Related Art
Conventionally, regenerative cryocoolers such as pulse tube cryocoolers or GM (Gifford-McMahon) cryocoolers have been used in order to cool apparatuses requiring cryogenic temperature atmospheres such as MRI (magnetic resonance imaging) apparatuses.
In the regenerative cryocooler, it is possible to form a state from the cryogenic temperature of approximately 4 K to the low temperature of approximately 100 K. For example, in the pulse tube cryocooler, a cooling effect is formed at low temperature ends of the regenerator tube and the pulse tube by repeating an operation for letting coolant gas compressed by a gas compressor flow in the regenerator tube and the pulse tube and an operation for receiving working gas in the gas compressor by letting the gas out from the regenerator tube and the pulse tube. In addition, by making these low temperature ends come in thermal contact with the subject to be cooled, it is possible to remove heat from the subject to be cooled.
The regenerator tube of the pulse tube cryocooler has a cylinder. A regenerative material fills the cylinder. The pulse tube is formed by a hollow cylinder. These cylinders have high temperature ends and low temperature ends. A cooling stage is provided at the low temperature end of the cylinder. The subject to be cooled is connected to this cooling stage.
If the amount of heat entering from the peripheral atmosphere to the high temperature end of the cylinder becomes large, the temperature at the low temperature end rises so that the cooling capacity of the cryocooler is degraded.
Because of this, in order to reduce heat transfer between the peripheral atmosphere and the high temperature end, it is suggested to make the walls of the cylinders forming the regenerator tube and the pulse tube thin.
In a case where the cylinders are made thinner, the cylinder is stretched in an axial direction due to repeating of compression and expansion of the coolant gas flowing inside. As a result of this, the cooling stage connected to the low temperature end of the cylinder may be vibrated.
In addition, due to such vibration of the cooling stage, a position of the subject to be cooled provided at the cooling stage may be changed. The change of the position of the subject to be cooled may be a serious problem in an apparatus such as a semiconductor manufacturing apparatus requiring positioning with high precision.
Because of this, in order to prevent the position change of the subject to be cooled due to vibration of the cooling stage, it is suggested to provide a vibration prevention apparatus at the cryocooler such as the pulse tube cryocooler. See, for example, Japanese Laid-Open Patent Application No. 2005-24184.
The vibration prevention apparatus discussed in Japanese Laid-Open Patent Application No. 2005-24184 has a second flange configured to support a subject to be cooled, separate from a normal support flange configured to support cylinders of the pulse tube and the regenerator tube.
A low vibration stage coming in contact with the subject to be cooled is supported at the second flange via a supporting stick. In addition, the cooling stage and the low vibration stage are thermally and mechanically connected to each other via a heat link, namely a flexible high thermal conductivity wire. The normal support flange and the second flange are connected to each other via a vibration absorption member such as a bellows. Because of such a vibration prevention apparatus, it is possible to prevent the vibration generated by the cylinder of the cryocooler from being transferred to the subject to be cooled.
However, in a case where the vibration prevention apparatus discussed in Japanese Laid-Open Patent Application No. 2005-24184 is provided, since the heat link is a wire having a small cross section, even if the heat link is made of a high thermal conductivity wire such as copper or aluminum, it is not possible to completely remove thermal resistance generated at the heat link.
Accordingly, in a method for thermally connecting the cooling stage and the low vibration stage (or the subject to be cooled) via the heat link, considering a case where the cooling stage and the subject to be cooled directly come in contact with each other, the cooling capacity of the cryocooler may be degraded.
In addition, if the vibration prevention apparatus is additionally provided, the structure of the cryocooler may be complex so that it is difficult to make the size of the cryocooler small.